A comparative study of the magnetic properties of bulk and nanocrystalline Co₃O₄

Abstract

A comparative study of the magnetic and electron paramagnetic resonance (EPR) parameters of bulk and Co3O4 nanoparticles (NP), synthesized by a sol–gel process, is presented. Both samples possess the cubic phase with a slightly lower (by 0.34%) lattice parameter for the Co3O4 NP. The average crystallite size D = 17 nm determined by x-ray diffraction (XRD) for the Co3O4 NP is quite consistent with the electron microscopic observations. The bulk Co3O4 has particle size in the 1–2 µm range. A Neel temperature of TN = 30 K (lower than the 40 K usually quoted in the literature) is determined from the analysis of the magnetic susceptibility versus temperature data for bulk Co3O4. This TN = 30 K is in excellent agreement with the TN = 29.92 K reported from specific heat measurements. The Co3O4 NP powder exhibits a still lower TN = 26 K, possibly due to the associated finite size effects. The values of coercivity, Hc = 250 Oe, and exchange bias, He = −350 Oe, together with the training effect have been observed in the Co3O4 NP sample (cooled in 20 kOe). Both Hc and He approach zero as . For T>TN, the χ versus T data for both samples fit the modified Curie–Weiss law (χ = χ0+C/(T+θ)). The magnitudes of C, θ and TN are used to determine the following: exchange constants J1ex = 11.7 K, J2ex = 2.3 K, and magnetic moment per Co2+ ion μ = 4.27 μB for bulk Co3O4; and J1ex = 11.5 K, J2ex = 2.3 K and μ = 4.09 μB for Co3O4 NP. EPR yields a single Lorentzian line near g = 2.18 in both samples but with a linewidth ΔH that is larger for the Co3O4 NP. Details of the temperature dependence of ΔH, line intensity I0, and disappearance of the EPR on approach to TN are different for the two samples. These effects are discussed in terms of spin–phonon interaction and additional surface anisotropy present in Co3O4 NP.